This invention generally relates to the field of radio frequency (RF) receivers and, more particularly, to a method and apparatus for producing complex baseband signals, also known as quadrature baseband signals from a wideband IF signal.
In order to recover modulated information, for example, audio information, radio receivers process received RF signals using well known demodulation techniques, such as frequency, amplitude or phase demodulation techniques. With the advent of powerful digital signal processors (DSPs), modern radio receivers demodulate the received signal more effectively using data processing techniques that rely on numeric representations of the received signal. In order to process a received signal, most digital demodulation techniques produce a complex baseband signal having complex components. These complex components, which are known as inphase (I) and quadrature (Q) components, carry amplitude, phase, and frequency information of the received signal, which allow a receiver's DSP to recover the modulated information by processing the numeric representations of the I and Q components.
There are a variety of known methods for producing the complex components of the received signal. One known method converts a received signal into its complex components by mixing an IF signal with a set of receiver generated reference signals that are 90 degrees out of phase from each other. Separate mixers mix the IF signal with the reference signals, which are also known as sine and cosine signals, to produce a complex baseband signal that comprise the I and Q components of the IF signal.
Generally, the mixers are fabricated using CMOS integrated circuit technology. Because of fabrication mismatches inherent in CMOS technology, the mixers introduce DC offset voltages at their corresponding outputs even when no signal is applied at their inputs. These DC offset voltages could reduce the dynamic ranges of the mixers in a way that adversely affects the digital signal processing resolution. Some conventional receivers include automatic gain control (AGC) circuits at the mixers' inputs to maintain the mixers' outputs within an optimum range. However, because most of the received RF signals are subjected to random variations, generally caused by objects within their propagation paths, the amplification level of the AGC circuits may not be predicted accurately. Therefore, implementation of the AGC circuit in the radio receiver becomes extremely complicated.
Another conventional method relies on phase information contained in a normalized IF signal and the amplitude of the received signal for providing the I and Q components. This Log-polar method is described in U.S. Pat. No. 5,048,059 which is hereby incorporated by reference. After down converting the received signal to an intermediate frequency, a radio receiver incorporating this method limits the IF signal using a limiter that includes cascaded amplification stages, which produce the normalized IF signal at a last amplification stage. At each stage, a detector detects the output levels of its corresponding stage. The output levels from all of the cascaded stages are summed with each other to produce a logarithmic representation of the IF signal amplitude. At the same time, the saturated output at the last stage of the limiter, which has a square waveform containing the phase information, is applied to a phase detector that detects the phase of the normalized IF signal. Based on the phase and amplitude of the IF signal, the receiver's DSP determines the I and Q components by converting the phase and amplitude information from a polar coordinate system to a Cartesian coordinate system.
The phase detector under the conventional method determines phase variations by detecting the durations of the zero-crossings of the normalized IF signal relative to a reference signal. The zero-crossing durations are detected by sampling the normalized IF signal at a predetermined rate, which corresponds to the resolution with which the phase variations are detected. By increasing the sampling rate, the phase detector detects the phase variations with finer resolution. For example, in order to produce a one degree phase resolution, the sampling rate must be 360 times the intermediate frequency. As a result, for producing acceptable phase resolution, the sampling rate under this method is substantially higher than the IF signal frequency.
With the introduction of wide band radio receivers, such as those based on Code Division Multiple Access (CDMA) techniques, the IF signal frequency of a CDMA radio receiver may be in 5-10 MHz range. Therefore, detecting the phase variations of a normalized wide band IF signal using the above described conventional technique requires a high sampling rate that can be provided by a costly high frequency clocking circuit that draws a substantial amount of current. In a battery operated portable radio receiver, which has a limited current source, the high current drain of such a clocking circuit becomes a limiting factor for using the conventional phase detector to provide the I and Q components of a wide band IF signal. Therefore, there exists a need for a cost effective way of producing the I and Q components of a wide band IF signal without drawing the substantial current required by high frequency clocking circuits.